3 Domain Names .. Single Account

A client of mine, for whom we are setting up Zimbra for has a few hundred email addresses. Each email address has to appear in 3 domain names (domain.com, domain.net, domain.org). The only way I can think to do this is to create the account on one of the domain names and then create 2 aliases for that account, one for each of the other domain names.

Now, I can script this up and it wouldn't be all that hard during the initial setup, but the customer is going to get tired of doing the aliases manually for each account.

Is there any way I can set this up in a way that's easier? Some directive that says domain.net and domain.org are just aliases to domain.com? This way the additional aliases are implied and don't have to be created?